Laser-based treatment of tissue is used for a variety of applications, such as hair removal, skin rejuvenation, wrinkle treatment, acne treatment, treatment of vascular lesions (e.g., spider veins, diffuse redness, etc.), treatment of cellulite, treatment of pigmented legions (e.g., age spots, sun spots, moles, etc.), tattoo removal, and various other treatments. Such treatments generally include delivering laser radiation to an area of tissue on a person's body, e.g., the skin or internal tissue, to treat the tissue in a photochemical, photobiological, thermal, or other manner, which can be ablative or non-ablative, among other properties, depending on the particular application.
Laser-based treatment devices may include any suitable type of laser, e.g., laser diode, fiber laser, VCSEL (Vertical Cavity Surface Emitting Laser), LED, etc. A device may include a single laser or multiple lasers, e.g., a laser diode bar including multiple distinct emitters arranged in a row, or multiple fiber lasers arranged in a row or array.
Diode lasers are particularly suitable for certain treatments and devices for providing such treatments. For example, diode lasers are compact, as they are typically built on one chip that contains all necessary components. Further, diode lasers typically provide an efficiency of up to 50%, which enables them to be driven by low electrical power compared to certain other lasers. Further, diode lasers allow direct excitation with small electric currents, such that conventional transistor based circuits can be used to power the laser.
Other characteristics of diode lasers include high temperature sensitivity/tunability, and a highly divergent beam compared to certain other lasers. Diode lasers typically emit a beam having an axis-asymmetric profile in a plane transverse to the optical axis of the laser. In particular, the emitted beam diverges significantly faster in a first axis (referred to as the “fast axis”) than in an orthogonal second axis (referred to as the “slow axis”). In contrast, other types of lasers, e.g., fiber lasers, typically emit a beam having an axis-symmetric profile in the transverse plane.
Laser-based treatment devices include larger-scale devices typically operated by a physician or other professional in a clinic or other office, as well as hand-held devices for home-use, allowing users to provide treatment to themselves. Some hand-held laser-based treatment devices are battery powered, e.g., using a Li ion battery cell (or multiple cells). Such battery-powered devices may be recharged between use, e.g., by plugging into an A/C wall outlet, either directly or by docking in a docking unit plugged into the wall.
Laser-based treatment devices typically provide a laser package for mounting the laser(s), providing power to the laser(s), and removing heat generated by the laser(s), e.g., by providing a thermal coupling to a heat sink or other thermal system. Single-emitter laser diodes and laser diode bars are commonly mounted on carriers or submounts that have a relatively low coefficient of thermal expansion (CTE) to match the CTE of the semiconductor material of the laser, while also exhibiting good thermal conductivity. Two common materials that are CTE-matched to the laser semiconductor, and provide good thermal conductivity, are CuW and BeO. These materials reduce or minimize stress on the laser semiconductor that can cause undesirable optical properties, such as polarization changes, as well as undesirable physical properties such as bar smile, wavelength shift, and bar cracking Bar cracking typically results in complete failure of the laser. The criticality of changes in polarization, bar smile, and wavelength shift depend on the particular application. These materials (CuW and BeO) are generally not considered low cost due to their intensive fabrication process.
Further, problems exist with certain conventional laser-submount solder connections. For example, a soft solder like Indium is commonly used to act as a compliant layer between a semiconductor laser and the carrier/submount. However, Indium solders may exhibit thermal and electro migration (e.g., the solder material may physically flow out from the solder joint, and may block the laser emitter(s) and thus result in device failure), especially in pulsed laser operations, e.g., employing multi-hundred millisecond pulses.